Children were a potential threat because they were used by the Taliban to assist in attacks against coalition forces, Marion Carrington said. Photograph: Adek Berry/AFP

The US military is facing fresh questions over its targeting policy in Afghanistan after a senior army officer suggested that troops were on the lookout for "children with potential hostile intent".

In comments which legal experts and campaigners described as "deeply troubling", army Lt Col Marion Carrington told the Marine Corp Times that children, as well as "military-age males", had been identified as a potential threat because some were being used by the Taliban to assist in attacks against Afghan and coalition forces.

The piece also quoted an unnamed marine corps official who questioned the "innocence" of Afghan children, particularly three who were killed in a US rocket strike in October. Last month, the New York Times quoted local officials who said Borjan, 12, Sardar Wali, 10, and Khan Bibi, eight, from Helmand's Nawa district had been killed while gathering dung for fuel.

However, the US official claimed that, before they called for the strike on suspected insurgents planting improvised explosive devices, marines had seen the children digging a hole in a dirt road and that "the Taliban may have recruited the children to carry out the mission".

But the apparent widening of the US military's already controversial targeting policy has alarmed human rights lawyers and campaigners.

Amos Guiora, a law professor at the University of Utah specialising in counter-terrorism, said Carrington's remarks reflected the shifting definitions of legitimate military targets within the Obama administration.

Guiora, who spent years in the Israel Defence Forces, including time as a legal adviser in the Gaza Strip, said: "I have great respect for people who put themselves in harm's way. Carrington is probably a great guy, but he is articulating a deeply troubling policy adopted by the Obama administration.

"The decision about who you consider a legitimate target is less defined by your conduct than the conduct of the people or category of people which you are assigned to belong to … That is beyond troubling. It is also illegal and immoral."

Guiora added: "If you are looking to create a paradigm where you increase the 'aperture' – that scares me. It doesn't work, operationally, morally or practically."

Guiora cited comments made by John Brennan, the White House counter-terrorism chief, in April, in which he "talked about flexible definitions of imminent threat."

Pardiss Kebriaei, senior attorney of the Center for Constitutional Rights and a specialist in targeted killings, said she was concerned over what seemed to be an attempt to justify the killing of children.

Kebriaei said: "This is one official quoted. I don't know if that standard is what they are using but the standard itself is troubling."

The US is already facing criticism for using the term term "military-aged male" to justify targeted killings where the identities of individuals are not known. Under the US definition, all fighting-age males killed in drone strikes are regarded as combatants and not civilians, unless there is explicit evidence to the contrary. This has the effect of significantly reducing the official tally of civilian deaths.

Kebriael said the definition was reportedly being used in Pakistan, Somalia and Yemen. "Under the rules of law you can only target civilians if they are directly participating in hostilities. So, here, this standard of presuming any military aged males in the vicinity of a war zone are militants, already goes beyond what the law allows.

"When you get to the suggestion that children with potentially hostile intent may be perceived to be legitimate targets is deeply troubling and unlawful."

Children in conflict zones have additional protections under the law.

Kebriael, who is counsel for CCR in a lawsuit which seeks accountability for the killing of three American citizens – including a 16 year old boy – in US drone strikes in Yemen last year, said that the piece also raised questions over how those killed in that incident were counted. "Were they counted as military-aged males or were they counted as children with potentially hostile intent or were they counted as the innocent bystanders they were?"

In a speech in April setting out the context for the US programme of targeted killings, White House counter-terrorism chief John Brennan spoke about a threshold of "significant threat', which was widely seen as introducing a lower criteria than "imminent threat".

Brennan said: "Even if it is lawful to pursue a specific member of al-Qaida, we ask ourselves whether that individual's activities rise to a certain threshold for action, and whether taking action will, in fact, enhance our security. For example, when considering lethal force we ask ourselves whether the individual poses a significant threat to US interests. This is absolutely critical, and it goes to the very essence of why we take this kind of exceptional action."

An Isaf spokesman, Lt Col Jimmie Cummings, told the Marine Corp Times that insurgents continue to use children as suicide bombers and IED emplacers, even though Taliban leader Mullah Omar has ordered them to stop harming civilians.